Open grasslands in California and Washington are being invaded by two closely related European shrubs, French broom and Scotch broom, that are considered among the most invasive and damaging of wildland species in those habitats. In this study, we present evidence of their effects on soil nitrogen (N) and the implications for restoration. Using natural abundance 15N signatures of leaves, we show that N fixation by brooms varies across sites and may depend on a suite of site-specific factors. Nonetheless, in sites in both California and Washington, we observe up to a twofold increase in soil N availability, as assayed in the laboratory. Across a range of sites, we determined that burning decreases total soil N by nearly 40%. We found burning to have the simultaneous effect of decreasing the broom seedbank by 68% after one burn. In a separate experiment, we removed broom and added sawdust to the soil to test whether a N-immobilization effect would help slower growing native perennial grasses in competition with European annual grasses. We found that although sawdust effectively decreased N availability after a 2-yr application period, we could not effectively target which group of species would benefit most.
Nomenclature: French broom, Genista monspessulana L. Link; Scotch broom, Cytisus scoparius L. Link.
Additional index words: Grassland, nitrogen fixation, restoration, shrub.